When fabricating a carton from a paperboard blank opposite ends of the blank are conventionally attached to each other by glue or by a mechanical lock to form a panel of the carton. In the case of a wrap-around carrier flaps at the ends of the blank typically are overlapped and connected by mechanical locks to form the bottom panel of the carrier. Since the bottom panel cannot be permitted to fail, it is essential that the locking system be capable of resisting forces caused by the weight of the packaged articles and by the stresses of shipping and handling which tend to pull the lock apart.
One approach to this problem utilizes primary and secondary locks. The primary locks connect the flaps together, while the secondary locks function to hold the flaps in place in order to prevent the primary locks from separating. Since it is essential to have strong secondary locks a number of systems employ rather large secondary male locking tabs, the fastening procedure of which typically requires the tabs to be located at the ends of the blank, extending out beyond the end edges of the blank. This is undesirable because the end location of the tabs does not permit the most economical blank layout during manufacture of the blanks.
Other designs provide secondary male locking tabs which are located entirely within the confines of a rectangular blank, but because of design constraints it is often difficult to adequately prevent withdrawal of the tabs from a carrier panel formed from overlapping flaps. A simple pull on the portion of the tabs extending into the interior of the carrier or the friction between the exterior portion of the tabs as the carriers slide across the packaging machine support surface can at times cause the tabs to separate and the bottom panel to fail.
It would be highly desirable to have a mechanical locking system which does not require the secondary male locking tabs to extend beyond the end edges of the blank but which effectively locks the tabs in place and resists withdrawal of the tabs.